The present invention is directed to a process for preparing a branched diene elastomer via an anionic route. The invention is particularly applicable to obtaining a starred elastomer.
There are a number of processes for preparing branched diene polymers by anionic routes. For example, U.S. Pat. Nos. 3,692,874 and 3,244,664 disclose processes that utilize halogenosilanes as coupling agents to obtain such polymers. Likewise, U.S. Pat. No. 4,914,248 and European specification EP-A-260 325 provide processes which utilize polyepoxides. European patent specifications EP-A-207 562 and EP-A-423 571, disclose processes using hexatriene and a polyethynylbenzene. U.S. Pat. No. 4,523,628 discloses preparing a starred elastomer using halogenated phosphines.
U.S. Pat. No. 3,803,266 provides a single example directed to the preparation of a starred block copolymer of styrene and butadiene using triethylphosphite. The xe2x80x2266 Patent also discloses that trialkylphosphites, wherein each alkyl group has 1 to 10 carbon atoms, preferably 1 or 2 carbon atoms, and triarylphosphites, wherein each aryl group has 1 to 12 carbon atoms, may be used in preparing the starred block copolymer.
U.S. Pat. No. 4,539,136 discloses a trifunctional starring agent, a tris-(nonylphenyl)phosphite used to produce a block diene copolymer of a conjugated diene and a vinylaromatic monomer, via an anionic route. The block copolymer has a high proportion of starring. The reference provides that a wide range in the value of the molar ratio between the quantity of starring agent and that of the lithiated initiator is allowed.
The examples of the xe2x80x2136 Patent disclose first reacting an organometallic initiator, such as sec-butyllithium, in cyclohexane with styrene to obtain a polystyrene and then adding butadiene to the polymerization product. Once the butadiene has polymerized completely, the trifunctional starring agent is added to the block copolymer so obtained in an amount such that the ratio between the number of moles of the starring agent and the number of moles of the initiator (molar ratio) ranges from 0.19 to 1.05.
The inventors have surprisingly discovered an improved process for the preparation of a branched diene elastomer having a high proportion of starred chains.
Thus, the present invention provides a process for preparing a branched diene elastomer having a high proportion of starred chains comprising anionically polymenrizing at least one conjugated diene monomer having 4 to 12 carbons in a polymerization medium comprising a hydrocarbon solvent and an organometallic polymerization initiator wherein tri-(2,4-di-tert-butylphenyl)phosphite added as the starring agent to the polymerization medium during or at the end of the polymerizing in an amount such that the molar ratio of the starring agent to the initiator is between 0.1 and 1.5, wherein a branched diene elastomer having a high proportion of starred chains is produced.
In the present invention, xe2x80x9cdiene elastomerxe2x80x9d means a homopolymer or copolymer obtained at least in part from diene monomers, i.e., monomers having two carbon-carbon conjugated or non-conjugated double bonds.
In accordance with the invention, the diene elastomer prepared by the process may be entirely a homopolymer obtained by polymerization of a conjugated diene monomer having 4 to 12 carbons, or a copolymer obtained by copolymerization of one or more dienes conjugated between one another or with one or more vinylaromatic compounds having 8 to 20 carbons.
The present invention provides a process for preparing a branched diene elastomer having a high proportion of starred chains comprising anionically polymerizing at least one conjugated diene monomer having 4 to 12 carbons in a polymerization medium comprising a hydrocarbon solvent and an organometallic polymerization initiator, wherein tris-(2,4-di-tert-butylphenyl)phosphite is added as a starring agent to polymerization medium during or at the end of the polymerizing in an amount such that the molar ratio of the starring agent to the initiator is between 0.1 and 1.5, wherein a branched diene elastomer having a high proportion of starred chains is produced.
Prior to the present invention it was not known to use tris-(2,4-di-tert-butylphenyl)phosphite as a starring agent. Heretofore, tris-(2,4-di-tert-butylphenyl)phosphite has only been known as an agent which protects or stabilizes polymers. See, for example, European patent specification EPA 048 562.
In accordance with the invention, the diene elastomer may be a homopolymer obtained by polymerizing a conjugated diene monomer having 4 to 12 carbon atoms or a copolymer formed by copolymerization of one or more diene monomers with each other or with a vinylaromatic compound having 8 to 20 carbon atoms.
In accordance with the present invention, suitable conjugated dienes for use in the process include 1,3-butadiene (butadiene), 2-methyl-1,3-butadiene (isoprene), 2,3-diethyl-1,3-butadiene, 2-methyl-3-ethyl-1,3-butadiene, 2-methyl-3-isopropyl- 1,3-butadiene, an aryl-1,3-butadiene, 1,3-pentadiene, and 2,4-hexadiene. Butadiene and isoprene are preferred.
Suitable vinylaromatic compounds include styrene, ortho-methylstyrene, meta-methylstyrene or para-methylstyrene, the commercially available mixture xe2x80x9cvinyltoluenexe2x80x9d, para-tertiobutylstyrene, and divinylbenzene. Styrene is preferred.
Copolymers may contain between 99% and 20% by weight of diene units and between 1% and 80% by weight of vinylaromatic units.
Particularly preferred polymers include polybutadienes, especially those with a content of xe2x88x921,2 units between 8% and 80%; polyisoprenes; copolymers of butadiene-styrene; especially those having a styrene content between 4% and 50% by weight, more particularly between 20% and 40% by weight, a content of xe2x88x921,2 units of the butadiene portion between 8% and 65% and a content of trans-1,4 units between 30% and 80%; copolymers of butadiene-isoprene, especially those having an isoprene content between 5% and 90% by weight; and copolymers of isoprene-butadiene, especially those having an isoprene content between 5% and 90% by weight.
Suitable butadiene/styrene/isoprene copolymers include those having a styrene content between 5% and 50% by weight, more particularly between 10% and 40%, an isoprene content between 15% and 60% by weight, more particularly between 20% and 50%, and butadiene content between 5% and 50%, more particularly between 20% and 40% by weight. Said butadiene/styrene/isoprene copolymers have a content of xe2x88x921,2 units between 4% and 85% and a content of trans-1,4 units between 6% and 80% of the butadiene part, a content of xe2x88x921,2 units plus xe2x88x923,4 units of the isoprene part between 5% and 70%, and a content of trans-1,4 units of the isoprene part between 10% and 50%.
The hydrocarbon solvent used in the polymerization medium may be selected from among toluene, cyclohexane, methylcyclohexane, heptane, n-hexane, cyclopentane, and mixtures containing two of these solvents.
The organometallic initiator used to initiate polymerization is preferably an organolithium compound, more preferably n-butyllithium.
In accordance with the invention, the tris-(2,4-di-tert-butylphenyl)phosphite starring agent may be added during or at the end of the homopolymerization reaction of a conjugated diene monomer or the copolymerization reaction of the conjugated diene monomer with a vinylaromatic monomer.
The starring agent is added in an amount such that the molar ratio of starring agent to initiator is between 0.1 to 1.5, preferably between 0.2 to 0.5.
The homopolymerization or copolymerization reaction may be carried out either as a continuous or a discontinuous (batch) process.
In accordance with the invention, the polymerization medium may additionally comprise a polar compound selected from the group consisting of diethers, diamines, tetrahydrofurans, and tetrahydrofurfuryl ethers.
A preferred diamine is tetramethylethylenediamine. Preferred diethers are 1,2-diethoxyethane, 1,2-dimethoxyethane, while preferred tetrahydrofurfuryl ethers include the tetrahydrofurfurylmethylethers, tetrahydrofurfurylethylethers and tetrahydrofurfurylpropylethers. THF is a preferred tetrahydrofuran.
The aforesaid characteristics of the present invention and others will be better understood on reading the following non-limiting examples of the invention, which are presented for illustration purposes.
In the examples below, the Mooney viscosity ML (1+4) at 100xc2x0 C. is determined in accordance with Standard ASTM D-1646.
The viscosities provided for the obtained elastomers are inherent viscosities determined at a concentration of 1 g/l in toluene at 25xc2x0 C.
The size exclusion chromatography (SEC) technique was used to determine the fractions by weight of starred chains in samples of the elastomers. In this technique the macromolecules are separated physically according to their respective sizes in the swollen state, in columns filled with a porous stationary phase. SEC was carried out using a model xe2x80x9c150Cxe2x80x9d chromatograph marketed by xe2x80x9cWATERSxe2x80x9d using a bank of two xe2x80x9cWATERSxe2x80x9d columns, type xe2x80x9cSTYRAGEL HT 6 Exe2x80x9d.
The near-infra-red radiation technique was used to determine the microstructural characteristics relating to the branched elastomers obtained. This measurement utilized a spectrometer marketed by xe2x80x9cBRUKERxe2x80x9d.